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OS_APPLE means OS_MAC or OS_IOS. Since OS_IOS is disallowed in chrome/,
it makes more sense to use OS_MAC in chrome/ instead of OS_APPLE.

Replace existing OS_APPLE usage with OS_MAC, and add a presubmit to
check for OS_APPLE usage.

Change-Id: I539f062f5f82a93dcbd861501421977e4f027517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2365434Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800196}